The invention relates to apparatus for moving bed or fixed bed processes for the production of aromatic compounds, in particular for reforming. More particularly, it concerns the oxychlorination step during regeneration of a used catalyst and is intended to restore its initial catalytic performances.
The catalyst generally comprises a support (for example, formed from at least one refractory oxide, the support possibly also including one or more zeolites), at least one noble metal (preferably platinum), and preferably at least one promoter metal (for example tin or rhenium), at least one halogen and optionally one or more additional elements (such as alkalis, alkaline-earths, lanthanides, silicon, group IVB elements, non noble metals, group IIIA elements, etc.). Catalysts of this type contain platinum, for example, and at least one other metal deposited on a chlorinated alumina support. In general, such catalysts are used to convert naphthenic or paraffinic hydrocarbons, which can be transformed by dehydrocyclisation and/or dehydrogenation, for reforming or for the production of aromatic compounds (for example for the production of benzene, toluene, orthoneta- or para-xylenes). Such hydrocarbons originate from fractionation of crude oil by distillation or other transformation processes.
Such catalysts have been widely described in the literature.
One way of increasing the yields of such reforming or aromatic compound production processes is to reduce the operating pressures at which the different reactions of interest are carried out. As an example, reforming reactions were carried out at 40 bars 30 years ago; 20 years ago, at 15 bars. Today, reforming reactors usually operate at pressures of less than 10 bars, in particular in the range 3 to 8 bars.
The improvement in desirable reactions due to a reduction in pressure is accompanied by more rapid deactivation of the catalyst by coking. Coke, a high molecular weight compound constituted essentially by carbon and hydrogen, is deposited on the active sites of the catalyst. The H/C molar ratio of the coke formed varies from about 0.3 to 1.0. The carbon and hydrogen atoms form condensed polyaromatic structures with a variable degree of crystalline organisation, depending on the function and nature of the catalyst and the operating conditions of the reactors. While the selectivity of transformation of the hydrocarbons to coke is very low, the amounts of coke accumulated on the catalyst can be large. Typically, for fixed bed units, such amounts are in the range 2.0% to 20.0% to 25.5% by weight. For circulating bed units, such amounts are below 10.0% by weight.
Coke deposition, which is more rapid at low pressure, also requires more rapid regeneration of the catalyst. Current regeneration cycles have become as short as 2-3 days.
Our European patent EP-A-0 378 482 discloses a continuous process for regenerating a reforming or aromatic compound production catalyst which can overcome the inherent disadvantages of shorter and shorter cycles. One of the regeneration steps is oxychlorination of the catalyst. The present invention concerns this step.
In EP-A-0 378 482, the used catalyst slowly travels from top to bottom in a regeneration vessel where it meets, in succession, a first radial moving bed combustion zone, a second radial moving bed combustion zone, an axial moving bed oxychlorination zone and an axial moving bed calcining zone, and:
a) in the first combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reforming reactor, at a temperature in the range 350xc2x0 C. to 450xc2x0 C., using a combustion gas based on an inert gas circulating as a co-current to the catalyst, comprising 0.01% to 1% of oxygen by volume, the combustion gas originating from a zone for washing the gases from the combustion, oxychlorination and calcining steps;
b) in a second combustion zone, the catalyst is treated at a pressure of 3 to 8 bars, substantially equal to that in the first reactor, at a temperature which is higher by at least 20xc2x0 C. than the temperature in the first combustion zone, in the presence of gases originating from the first combustion zone and in the presence of an inert makeup gas to which up to 20% by volume of oxygen is added so that the catalyst is in contact with a gas comprising 0.01% to 1% by volume of oxygen, the gases circulating as a co-current with the catalyst;
c) the burn gases are evacuated from the second combustion zone and sent to a washing circuit after first being mixed with the gases extracted from the oxychlorination zone and the calcining zone;
d) in the axial oxychlorination zone, the catalyst is treated with a co-current of a mixture of a gas originating from the calcining zone and the chlorinated gas for 30 min to 60 min, the mixture forming an oxychlorination gas comprising 4% to 10% by volume of oxygen, at a pressure of 3 to 8 bars; the water content is of the order of 500-7000 ppm, with no added water, it originates from the gas from the combustion step, which has been washed and dried and used in part for oxychlorination, but also essentially for calcining;
e) in the axial calcining zone, the catalyst is treated for 45 min to 80 min in a counter-current at between 350xc2x0 C. and 550xc2x0 C. at a pressure in the range 3 to 8 bars, using a portion of the gas originating from the washing circuit and a drying zone, the gas not containing more than 100 ppm of water.
A number of patents concern the regeneration of such existing catalysts, in particular U.S. Pat. Nos. 4,980,325 and 5,053,371. In those patents, the oxychlorination and combustion zones are separate so as to allow the catalyst to pass but not gas, and there is a circuit for recycling the gases from the oxychlorination step. U.S. Pat. No. 5,053,371 describes the operating conditions: 3-25% of oxygen in the gas introduced into the oxychlorination step, a chlorine content in the oxychlorination zone of the order of 500 ppm molar and a low water content which originates from the catalyst and gas from the calcining step. in U.S. Pat. No. 4,980,325, the oxygen originates solely from the oxygen-enriched gas which is introduced to the calcining step.
We have established that while they re-introduce chlorine into the catalyst, those operating conditions for the oxychlorination step, do not ensure correct re-dispersion of the bimetallic phase. This results in a degradation of the catalytic action over time.
Thus, a gas management which could precisely control the operating conditions of the oxychlorination step and preferably also those of the oxychlorination step was researched.
The process and unit of the invention satisfy these objectives.
More precisely, the process of the invention is a process for regenerating a catalyst for aromatic hydrocarbon production or for reforming, the catalyst comprising a support, at least one noble metal and chlorine, the process comprising successive combustion, oxychlorination and calcining steps, in which process at least one chlorinating agent, at least one oxygen-containing gas and water are introduced into the oxychlorination step, such that the H2O/HCl molar ratio is 3 to 50, the oxychlorination step being carried out in the presence of an oxychlorination gas containing less than 21% of oxygen and at least 50 ppm by weight of chlorine (based on HCl), at a temperature of 350-600xc2x0 C., preferably 350-550xc2x0 C.
The process can be carried out in a fixed bed (the steps are then carried out successively in the same zone) or in a moving bed or with intermittent flow of the catalyst (in this case, each step is carried out in at least one different zone, the catalyst flowing from one zone to the other).
Regeneration starts with a step for combustion of the carbonized material. It is followed by an oxychlorination step then by a calcining step.
In general, the gases from the combustion step and the gases from the oxychlorination step are separately extracted from the regeneration process. In order to prevent the gases from mixing, a plate or other means is advantageously positioned so as to separate the combustion and oxychlorination zones in moving bed processes. In contrast, in these moving bed processes, gases from the calcining step can generally pass freely into the oxychlorination zone.
Whether the process is carried out in a fixed or moving bed, the catalyst which has undergone the combustion step is ready to undergo an oxychlorination step. It is carried out in one or more zones, of axial or radial type. At least one chlorinating agent, at least one oxygen-containing gas and water are introduced into the oxychlorination zone. The chlorinating agent can be chlorine, HCl, or a halogenated hydrocarbon containing less than 4 carbon atoms and 1 to 6 chlorine atoms (for example CCl4) or any chlorinating agent which is known to liberate chlorine in these regeneration processes. It is preferably introduced with the oxygen-containing gas. In moving bed processes, it is advantageously introduced into the lower portion of the oxychlorination zone so that it flows as a counter-current to the catalyst, when the oxychlorination zone is axial.
The quantity of chlorinating agent introduced is such that the chlorine concentration (based on HCl) in the gas in contact with the catalyst in the oxychlorination zone, termed the oxychlorination gas (i.e., for moving bed processes, the gas introduced into the oxychlorination zone+the gas originating from the calcining zone), is at least 50 ppm by weight, in general 50-8000 ppm by weight, advantageously more than 650 ppm by weight, and preferably in the range 1000 to 8000 ppm by weight. For technical reasons (linked to corrosion, for example, or to the subsequent treatment of the chlorinated gases), it is also preferable to operate with contents which do not exceed 4000 or 5000 ppm by weight.
At least one oxygen-containing gas is also introduced into the oxychlorination zone. This gas advantageously contains a portion of the gases from the combustion step, preferably washed and dried, with additional makeup oxygen, for example air. In moving bed processes with an axial oxychlorination zone, this gas preferably circulates as a counter-current to the catalyst.
In the oxychlorination zone, the catalyst is in contact with the gas introduced and also, for moving beds, in contact with gas originating from the calcining zone, charged again with oxygen and containing a little water from the calcining step. The oxygen content of the oxychlorination gas is below 21% (by volume). It is generally above 10% by volume.
It can be seen that in the invention, in a preferred moving bed process, and in contrast to the prior art EP-A-0 378 482, at least one oxygen-containing gas is introduced into the oxychlorination step (the axial oxychlorination zone, for example), independently of the oxygen-containing gas introduced into the calcining step (the axial calcining zone, for example).
Without departing from the scope of the invention, it is also possible for moving bed processes to introduce into the oxychlorination step only chlorinating agent and water, in which case good distribution of chlorine and water is more difficult to achieve, the oxygen-containing gas then originating only from the calcining zone.
In a novel advance over EP-A-0 378 482, water is introduced into the oxychlorination step. It is advantageously supplied as a mixture with the oxygen-containing gas introduced.
The quantity of water introduced is in an H2O/HCl molar ratio of 3 to 50, preferably 4 to 50, or 4 to 30, advantageously 7 to 50, and more preferably 7 to 30. Water is supplied in liquid form or, as is preferable, as steam.
The oxychlorination gas is thus highly charged with water, and its water content is over 7000 ppm, generally at least 8000 ppm or even 10000 ppm by weight, preferably over 10000 ppm by weight.
The noble metal is re-dispersed in the presence of oxygen, chlorine and water under the described conditions, and at temperatures of 350-600xc2x0 C., preferably 350-550xc2x0 C. in the oxychlorination step, but usually at least 450xc2x0 C., preferably between 490xc2x0 C. and 530xc2x0 C. The residence time of the catalyst in the oxychlorination step is normally less than 2 hours and is generally between 45 min and 2 hours.
The pressure in this zone must be balanced with the pressures in the adjacent zones when the catalyst is circulated, and at 3-8 bars for moving bed processes for catalyst regeneration operating in low pressure reforming processes.
In a preferred implementation of a moving bed process, the oxychlorination gas results from mixing the gas originating from the zone in which the calcining step is carried out with the chlorinating agent(s), water and the oxygen-containing gas(es) introduced into the zone in which the oxychlorination step is carried out, the oxygen-containing gas(es) comprising a portion of the gases from the combustion step with an additional oxygen makeup, and the gas introduced into the calcining zone is air or a gas formed from a portion of the gases from the combustion step which have been washed, dried and had an oxygen makeup.
In these moving bed processes, the oxychlorination gas also contains gas originating from the calcining zone; an oxygen-containing gas is introduced into this calcining zone, also less than 1 mole % of water, preferably less than 0.1% of water and more preferably less than 0.05% of water. In general, the water content will be below 150 ppm molar, preferably less than 100 ppm molar and advantageously less than 50 ppm molar.
The oxygen-containing gas can be air. Advantageously, this gas comprises a portion of the gas from the combustion step, which has been washed and dried, with an addition of oxygen (air). In this advantageous case, the oxygen content in the gas introduced into the calcining step is less than 21% by volume. In general, the oxygen content of the gas introduced into the calcining step is at most 21% by volume.
As is known, the temperature of the calcining step is in the range 350xc2x0 C. to 600xc2x0 C., preferably 350-550xc2x0 C. The oxygen-containing gas circulates as a counter-current to the catalyst in moving bed processes with an axial calcining zone. In general, the residence time is less than 1 hour.
In order to strictly control the operating conditions in the oxychlorination zone, it is preferable to operate without recycling the oxychlorination gases.
The absence of recycling also enables the oxygen content to be more precisely controlled, and means that high oxygen contents (no dilution) can economically be obtained.
However, some implementations may include recycling.
In the absence of recycling (preferred case), the oxychlorination gas (or the purge of this gas if it is recycled) leaving the oxychlorination zone is discharged from the unit (for example into the atmosphere) after treatment to eliminate at least the chlorinated impurities.
It is also important to dry the gas supplied to the oxychlorination zone from the combustion step, when this is the case, to control the quantity of water present in the oxychlorination gas using the quantity of water added. The gas extracted from the combustion step can be dried before it is fractionated to supply a portion to the oxychlorination zone, or the fractionated portion can be dried. The air is also preferably dried.
Under the conditions of the process of the invention, a considerable improvement in re-dispersion of the metallic phase of the catalyst is obtained with respect to the prior art, as will be shown in the example.
The state of dispersion of the metallic phase of the catalyst is quantitatively determined by H2/O2 chemisorption.
The invention also concerns a vessel for carrying out the process of the invention.
The vessel of the invention is a vessel for regenerating a catalyst for reforming or for aromatic compound production comprising a support, at least one noble metal and chlorine, the catalyst being in the form of a moving bed, said vessel comprising at least one combustion zone (A) provided with at least one conduit (9) for introducing oxygen-containing gas and at least one conduit (5) for evacuating gases from the combustion step, at least one oxychlorination zone (B) and at least one calcining zone (C) provided with at least one conduit (18) for introducing an oxygen-containing gas, said vessel also comprising at least one conduit (1) for introducing catalyst into the vessel, at least one conduit (3) for introducing catalyst from the combustion zone into the following oxychlorination zone (B), and at least one conduit (21) for evacuating gases from the oxychlorination step, the vessel being characterized in that the oxychlorination zone comprises at least one means (19) for introducing at least one chlorinating agent and at least one means (20) for introducing water and at least one means (17) for introducing an oxygen-containing gas.